Update message-io requirement from 0.17.0 to 0.18.0

Updates the requirements on [message-io](https://github.com/lemunozm/message-io) to permit the latest version.
- [Changelog](https://github.com/lemunozm/message-io/blob/master/CHANGELOG.md)
- [Commits](lemunozm/message-io@v0.17.0...v0.18.0)

---
updated-dependencies:
- dependency-name: message-io
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

proptest-state-machine/Cargo.toml

@@ -27,4 +27,4 @@ proptest = { version = "1.2.0", path = "../proptest", default-features = true, f
 ] }
 
 [dev-dependencies]
-message-io = "0.17.0"
+message-io = "0.18.0"